dust collection math help please

[Bob Riefer]

Assumptions:
I understand 6" is better than 4", and have read all the Pentz information about fine dust.. But at this time and budget, this is what I have to work with (and actually, I think this is what many hobbyists have to work with, so perhaps this exercise will be useful to others):

Budget budget budget... i.e. probably one of the HF models
4" PVC pipe for main and all branches
blast gates, one tool at a time
thien baffle sitting before the blower intake
exhausting outside (NO filter)
Almost all the tools in my shop need about 400 CFM
Longest run, (30 straight feet + 1 bend up, 1 bend over, 1 bend down) = 50 feet

Given this information, can anyone help me understand the math on selecting a dust collector? Most likely, due to budget, I am restricted to a HF model. I was thinking of the 2 hp model.

Thanks!

[Philip Rodriquez]

30 feet is not going to happen with a 2 HP unit. Sorry. Look up the information on static pressure (SP).

[Bob Riefer]

I started this thread to understand what it WILL take, not to state that I think 2 hp is enough (because I really don't know.... I'm having trouble understanding the math, hence this thread). Also, my longest run is 50 feet when you consider the bends.

[Anthony Whitesell]

According the test in Wood Magazine #182 the Central Machinery (HF) 2HP collector will pull 400CFM at 5.75SP and 536CFM at 4.25SP. Those are your starting points. I'll find the tables and get back with some math.

[David Nelson1]

Assumptions:
I understand 6" is better than 4", and have read all the Pentz information about fine dust.. But at this time and budget, this is what I have to work with (and actually, I think this is what many hobbyists have to work with, so perhaps this exercise will be useful to others):

Budget budget budget... i.e. probably one of the HF models
4" pipe for main and all branches
blast gates, one tool at a time
thien baffle sitting before the blower intake
exhausting outside (NO filter)
Almost all the tools in my shop need about 400 CFM
Longest run, (30 straight feet + 1 bend up, 1 bend over, 1 bend down) = 50 feet

Given this information, can anyone help me understand the math on selecting a dust collector? Most likely, due to budget, I am restricted to a HF model. I was thinking of the 2 hp model.

Thanks!

Wood E newletter that was just sent out is all about dust collection. I PM'ed you with my E address if your interested.

Anthony has posted the link but it has more....... From the begining: http://www.woodmagazine.com/woodworking-tips/techniques/dust-collection/top-dust-collection-tips/?esrc=nwwood&email=477101870

[Anthony Whitesell]

Check out this link for helping with the math.
http://www.woodmagazine.com/woodworking-tips/techniques/dust-collection/figure-dust-collection-needs/?page=1

Based on wood magazines numbers you need to stay below 5.75SP to maintain the 400CFM.

As for your set-up 30 ft 4" flex = 0.07SP/ft (@ 4kCFM)*30ft=2.1SP
Three 90 degree bends = 3 bends @ 6ft equivalent straight length = 0.07SP/ft*18ft=1.26SP
The tool hood will add another 1.8SP, but nothing for the filter (or lack there of)
According to Bill Pentz website, a trash can separator adds 4.5SP to the system (I believe this to be a typical number and not set in stone)

The total so far is atleast 5.16SP (to possibly as high as 7.86 depending on the trashcan separator). So according to this information you could be in trouble for getting the numbers (400CFM) that you expect.

According to the same Wood Mag article, the Delta 50-760, Grizzly G1029Z, and the Penn State DC2000B would fit have the performance required.

P.S. Please don't take my assessment as fact, I came up with the numbers based on my understanding. Please correct me where I've gone wrong.

[Bob Riefer]

I'm sorry, I should have been clearer - the straight runs are all 4" PVC (not flex) which I'm assuming improves performance a bit (I edited my original post to be clear). Thanks for the feedback, I'm going to dig into that link now :)

[Anthony Whitesell]

I used the 0.07SP/ft number from the Wood magazine website. I'm not sure that it is applicable to flex, though that is what I had typed. You are right though, PVC would be better than flex in terms of SP.

[Bob Riefer]

Alright, I tried following through the steps in the article.
http://www.woodmagazine.com/woodwork...-needs/?page=1 (http://www.woodmagazine.com/woodworking-tips/techniques/dust-collection/figure-dust-collection-needs/?page=1)

Here's a recap:

Summary:
You're seeking to determine duct size for the type of equipment that you have, as well as what specs blower you need (namely, how many CFM and what static pressure the unit can handle).

Step 1: How many CFM?
What is max CFM's needed for any of your tools? (article has a few ways to determine this... from what I can tell, most homeowner shops will be around 400 CFM)

Step 2: Duct size?
According to the article, 400 CFM would require a 4" pipe. (I know many will say 6" is the only way.. refer to my assumptions in the opening post).

Step 3: Static pressure
They said to figure out each branch + main to figure out your "worst run"... But I just intuitively looked at my drawing to see which of mine was longest.

Total length of worst run = rigid pipe feet + flex hose feet x 3 + 90 degree bends x 6 + 45 degree bends x 3

In my case = 30 + 1x3 + 3x6 + 0x3 = 49

Static Pressure Loss (for 4" pipe) = Total length x .07

In my case = 49 x .07 = 3.43 static pressure loss of worst branch

To Anthony's point, there's also the thien baffle to account for. Let's call that 4.55 SP. And the tool hood at another 1.8 SP.

SO.. grand total Static Pressure loss for worst run, 4" pipe, baffle, and tool hood = 3.43 + 4.55 + 1.8 = 9.78


Step 4: Pick a DC
With this information, you would be seeking a dust collector that is rated for at least 400 CFM at 9.78 static pressure loss.

edit: article with testing results on various 2 hp models: http://www.deltaportercable.com/uploads/PCD/Documents/News/182DustCollectors.pdf
As Athony noted, Delta 50-760 (110V), Grizzly G1029Z (220V), and PSI DC2000B (220V) seem to be in the ball park.


Did I do the math steps correctly?

[Anthony Whitesell]

And that would be a link the same article I was referring.

If you came up with 9.78 then none of the will work. The best is listed at 9.0 at 400CFM.

I haven't looked at DC setups this way. Interestingly enough that according to the links and math produced by the Wood magazine article 4" duct is 9.78, 5" duct is 9.54, and 6" duct is 9.365. Either I'm missing something or there is something funny with the article.

Using Bill Pentz staticcalc worksheet, is entered 1 hood, 3 90s, 30' straight and 3' of flex pipe, and 4.55 for the seperator and a target velocity of 4000FPS. For 4" pipe at 400 CFM it computed 9.79, 5" at 550 CFM (per the Wood Mag link) is 8.91, and 6" at 785 CFM is 8.34.

The aspect I find interesting is that the SP did not drop as much as I expected as the duct diameter increased.

P.S. 30+3+18=51 not 49. My comparisons here used 49 equivalent feet before it occurred to me.

[Bob Riefer]

If you came up with 9.78 then none of the will work. The best is listed at 9.0 at 400CFM.


It's the baffle (which is possibly a debateable number) and hood that are throwing the number over, and I'm over-estimating everything in the calculation.... So I'm thinking in reality I may be ok. I'm searching for information on how much loss might be expected from the thien baffle.

[David Nelson1]

Alright, I tried following through the steps in the article.
http://www.woodmagazine.com/woodwork...-needs/?page=1 (http://www.woodmagazine.com/woodworking-tips/techniques/dust-collection/figure-dust-collection-needs/?page=1)

Here's a recap:

Summary:
You're seeking to determine duct size for the type of equipment that you have, as well as what specs blower you need (namely, how many CFM and what static pressure the unit can handle).

Step 1: How many CFM?
What is max CFM's needed for any of your tools? (article has a few ways to determine this... from what I can tell, most homeowner shops will be around 400 CFM)

Step 2: Duct size?
According to the article, 400 CFM would require a 4" pipe. (I know many will say 6" is the only way.. refer to my assumptions in the opening post).

Step 3: Static pressure
They said to figure out each branch + main to figure out your "worst run"... But I just intuitively looked at my drawing to see which of mine was longest.

Total length of worst run = rigid pipe feet + flex hose feet x 3 + 90 degree bends x 6 + 45 degree bends x 3

In my case = 30 + 1x3 + 3x6 + 0x3 = 49

Static Pressure Loss (for 4" pipe) = Total length x .07

In my case = 49 x .07 = 3.43 static pressure loss of worst branch

To Anthony's point, there's also the thien baffle to account for. Let's call that 4.55 SP. And the tool hood at another 1.8 SP.

SO.. grand total Static Pressure loss for worst run, 4" pipe, baffle, and tool hood = 3.43 + 4.55 + 1.8 = 9.78


Step 4: Pick a DC
With this information, you would be seeking a dust collector that is rated for at least 400 CFM at 9.78 static pressure loss.

edit: article with testing results on various 2 hp models: http://www.deltaportercable.com/uploads/PCD/Documents/News/182DustCollectors.pdf
As Athony noted, Delta 50-760 (110V), Grizzly G1029Z (220V), and PSI DC2000B (220V) seem to be in the ball park.


Did I do the math steps correctly?

Recalculate using the CFM 1550 and a max static of 11 of the Grizzly and take the 2 four inch y's out and replace it with 6 inch and see whare you are then.

[Anthony Whitesell]

Recalculate using the CFM 1550 and a max static of 11 of the Grizzly and take the 2 four inch y's out and replace it with 6 inch and see whare you are then.

Those numbers are nice for the advertisement, but don't mean as much as they should. 1550CFM is only achievable at 0"SP and 11"SP at 0CFM. What is needed is the fan curve or the SP at the intermediate CFMs (like 400, 550, and 785 for 4, 5, and 6 inch duct). Wood magazine attempted to produce one in the article. Using the fan performance, the CFM can be determined for the SP of the system.

[Anthony Whitesell]

I concur with that assessment. I wonder if anyone can provide the requirements/setup for testing how much SP the separator adds to the system.

[David Nelson1]

Those numbers are nice for the advertisement, but don't mean as much as they should. 1550CFM is only achievable at 0"SP and 11"SP at 0CFM. What is needed is the fan curve or the SP at the intermediate CFMs (like 400, 550, and 785 for 4, 5, and 6 inch duct). Wood magazine attempted to produce one in the article. Using the fan performance, the CFM can be determined for the SP of the system.

LOL I'm still stuck with a 650 unit and very very short hoses.

[Anthony Whitesell]

Not to highjack, but personally I'm stuck between in small shop or a closet (ie., between a rock and a hard place hahaha). I have to sacrifice floor space for a larger stationary DC because I don't have the floor/aisle space to roll around a DC to keep the runs short.

[Heath Brandenburg]

An article from Wood Magazine (March 2008) in which they tested and plotted the performance curves of various 1.5 - 2 HP DCs:
http://www.deltaportercable.com/uploads/PCD/Documents/News/182DustCollectors.pdf

[Charlie Stone]

Just get 2 small HF models, put one on each end of the shop and be done with it. Math geeks ... sheesh

[Anthony Whitesell]

How did you know?

[Charlie Stone]

I am married to one

[Bob Riefer]

math geeks! ha ha, nice. (my mother is a mathmetician actually... me? math challenged... obviously :) )

[David Giles]

It's the baffle (which is possibly a debateable number) and hood that are throwing the number over, and I'm over-estimating everything in the calculation.... So I'm thinking in reality I may be ok. I'm searching for information on how much loss might be expected from the thien baffle.

No, you are not over estimating. Your calculations are right on track with my experience. I started my DC system just like you are doing. Lots of calculations and little experience. Ended up with a Jet 1.5Hp with canister. 5-6 "improvement" included an external Thien baffle, an internal Thein baffle, upgraded headers from 4" to 6", moving to external dust discharge and canister elimination, increasing machine ports to 6". Here's what I learned:
1. Decide if you want to be a woodworker or a dust collection expert. I regret the time spent in multiple DC modifications.
2. Bigger is better. Bigger blowers, motors, duct work mean bigger air flow volumes.
3. Vent outside if at all possible (as you are planning to do).
4. Just put in the cyclone to start with and get on with woodworking!

Ultimately, I installed an ebay cyclone with the Jet blower/motor. It works adequately well with 6" ductwork and external venting.

We get lost in the details of DC calculations and overlook the basics. Smaller blower/motor need larger ducts to compensate. But 4" PVC is cheap to replace later.

[David Giles]

I concur with that assessment. I wonder if anyone can provide the requirements/setup for testing how much SP the separator adds to the system.

Let's take a reasonable guess.
4" PVC pipe with 400 cfm creates about 70 fps air velocity. One velocity head loss is 1.2" WG.

90 degree ell to enter trash can separator: 0.5 VH
Entrance 45 degree ell: .25 VH
Exit Loss: 1.0 VH
Swirling around inside trash can: ignore
Entrance loss to leave trash can lid: 0.5 VH
90 degree ell after trash can separator: 0.5 VH
Piping length loss: ignore
-----------------------------
Total trash can separator pressure loss: 2.75 VH
x 1.2" WG per VH = 3.3" WG total.

Pretty close to Bill's 4.5" WG loss at his website. It's significant.

[Bob Riefer]

Hi David, Thanks for jumping in with your comments. Very useful information that you provided!


---some additional comments (not specifically to David, just in general)---

I'm of the mindset that a cheap, imperfect solution now is preferable to a much better solution that cannot be implemented for quite some time (possibly multiple years) due to budget constraints. I have free 4" PVC, a free 50 galon drum, and see HF 2HP DC's "working well" for lots of people (sure, not scientific studies, can't trust forums, etc. etc. but still.....) for $139. That's a solution that can be implemented next week.

Also, I would intuitively say that the "worst branch" approach creates possibly a false sense of doom and gloom. In my case, as an example, my worst run (band saw) is about twice as long as my average run, and about 3 times as long as my shortest runs (jointer, table saw, miter). So, while band saw dust collection may be less than optimal, all these other areas are likely to be much better.

I'm ok with a respirator, broom, and the ocassional leaf blow out of the shop as part of my dust collection solution given the budget constraints.

---
additional questions

In my case, I can make my bandsaw or my drill press be at the end of my "worst run". If it's your shop, which of those would you like to have closer to the DC? (I'm thinking that the band saw may be the messier of the two and should be closer to the DC)

Is there benefit to placing the thien baffle after the blower (instead of between blower and tools) in terms of reducing the static pressure loss? Any negative side effects of that decision?

[David Giles]

Jointers and planers are the easiest for chip collection. The table saw is the most difficult. Much of the dust goes over the top of the saw and the high velocity blade creates high velocity dust that is difficult for low velocity air flows to capture. Routers are even worse. I just sweep up after a routing operation. A 4" connection on a Grizzly bandsaw is more than adequate. The air flow swirls inside the lower chamber keeping it clean. The BS doesn't create high velocity dust, so it is easier to capture. No experience with a drill press, but it should be easy once the shavings are actually in the pipe. Getting them into the pipe is the problem.

Since you are venting outside, I assume that you plan to eliminate the dust bag or filter. That makes a big improvement in air flow. But I would recommend the ebay cyclone over the Thien trashcan system. Probably the same pressure drop and a pushthrough system fits well with outside venting. I can tell you, for certain, that a Thien system on a standard DC with the upper bag / canister removed works poorly. And the Thien systems are easily plugged by paper towels and long, thin wood strips if you use the DC as a shop vac.

[Anthony Whitesell]

Don't get me wrong, I didn't doubt it was significant. I was just wondering how far off the 4.5 might be. I guess not far off.

What's VH stand for?

[Phil Thien]

3. Vent outside if at all possible (as you are planning to do).
4. Just put in the cyclone to start with and get on with woodworking!

Ultimately, I installed an ebay cyclone with the Jet blower/motor. It works adequately well with 6" ductwork and external venting.

We get lost in the details of DC calculations and overlook the basics. Smaller blower/motor need larger ducts to compensate. But 4" PVC is cheap to replace later.

Just going to mention again (sorry) that #3 and #4 seem contradictory to me. I suppose if you don't want to go through the trouble of cutting/installing a baffle in your ring, it makes sense. But using a cyclone instead of a ring modded w/ a baffle when you're going to exhaust outside adds: (1) Bulk and (2) Bucks.

[Phil Thien]

Let's take a reasonable guess.
4" PVC pipe with 400 cfm creates about 70 fps air velocity. One velocity head loss is 1.2" WG.

90 degree ell to enter trash can separator: 0.5 VH
Entrance 45 degree ell: .25 VH
Exit Loss: 1.0 VH
Swirling around inside trash can: ignore
Entrance loss to leave trash can lid: 0.5 VH
90 degree ell after trash can separator: 0.5 VH
Piping length loss: ignore
-----------------------------
Total trash can separator pressure loss: 2.75 VH
x 1.2" WG per VH = 3.3" WG total.

Pretty close to Bill's 4.5" WG loss at his website. It's significant.

So if you work your #'s backwards to try to compute the pressure loss of a conventional cyclone (by subtracting the values I've highlighted in red because a conventional cyclone doesn't use them), you arrive at a pressure loss of 1.0 VH? That doesn't make any sense.

Here is a link to an interesting article:

http://onlinelibrary.wiley.com/doi/10.1002/ceat.200402089/abstract?systemMessage=Due+to+scheduled+maintenanc e+access+to+the+Wiley+Online+Library+may+be+disrup ted+as+follows%3A+Saturday%2C+30+October+-+New+York+0500+EDT+to+0700+EDT%3B+London+1000+BST+ to+1200+BST%3B+Singapore+1700+SGT+to+1900+SGT.

You don't need to read the entire article, the abstract says it all:

Based on investigation of the flow pattern in cyclone separators, a new theoretical model was developed for the prediction of pressure drop across cyclone separators. This model includes the effect of the geometrical dimensions and flow parameters, and assumes that the total pressure drop consists of four main partial pressure drops due to gas expansion at the separator entrance, wall friction within the separator, swirling motion of the gas, and gas flow through the outlet pipe. The availability of the method is verified by the comparison of calculated values with experimental data and with other models for different dimensions of cyclones.
The part where you ignore swirling around inside the can is the major problem in your #'s. You just can't ignore that.

And considering that many users build my separator using tangential inlets, or (even easier) modify the rings of their DC w/ a baffle, it isn't unreasonable to think that the pressure loss of baffle-based separators may well be lower than most conventional cyclones.

[Craig Carpenter]

Just last night I was searching for a collector system that I might be able to use on my job sites when trimming out larger homes that require many different peices of equipment on site. With 5 - 10 Carpenters working the same house, you can imagine the amount of dust floating around. Jobsite equipment comprises of many Miter saws, planer/jointer, 2 table saws, band saw, not to mention belt, orbital and finish sanders.

I came across this article written by a man who has a home shop. I found it to be very interesting and a wake up call as far as how fine dust particles can affect our health. While not trying or meaning to hijack your thread, I'm merely trying to bring this to the attention of other woodworkers and give you a few ideas for the placement of your collection system. I hope all who read it will appreciate what the man did to protect his health and the health of his family.

http://billpentz.com/woodworking/cyclone/index.cfm

Granted, the Author is selling his own system, but the health issues he discusses seem to be valid on the surface. I have a friend (Carpenter) that has been forced into early retirement at 61 yrs old due to COPD. The article as a whole brought more awareness to me as a Carpenter/Woodworker to the dangers of sawdust.

[David Giles]

What's VH stand for?
Sorry. VH stands for "velocity head". Just like a car, it takes energy get air up to speed. 70 ft/sec air velocity is about 50 mph. One VH is the energy needed to accelerate air from zero to actual pipe velocity. The exact equation (for sealevel folks) is VH (inch WG) = 0.000228 X (vel)squared.

A simplistic method of analyzing a ductwork system is to count up the total velocity head losses. Now you can easily calculate the total system pressure drop and match that number to your fan curve.

Every elbow, tee or pipe restriction incurs a VH loss. For example, if the air exits an opened pipe, it loses all of the energy contained in the velocity. So the VH loss = 1. Entering a sharp edged pipe has a VH loss = 0.5. Good engineers have complicated equations for accurate analysis. I have a book of pictures and corresponding VH loss numbers.

Fluid flow is not a difficult subject, but there are so many ways to approach it that we often talk past each other.

[David Giles]

So if you work your #'s backwards to try to compute the pressure loss of a conventional cyclone (by subtracting the values I've highlighted in red because a conventional cyclone doesn't use them), you arrive at a pressure loss of 1.0 VH? .....

And considering that many users build my separator using tangential inlets, or (even easier) modify the rings of their DC w/ a baffle, it isn't unreasonable to think that the pressure loss of baffle-based separators may well be lower than most conventional cyclones.

Phil, I love your baffle idea and have installed two different versions. For an existing dust collector, I don't see why the Thien baffle would add any pressure drop to the overall system. It is a wonderfully simple method of improving dust collection in a commonly used piece of equipment.

My point is that adding a separate trashcan separator in front of an existing DC adds lots of pressure drop. Adding a cyclone in front of an existing DC would also increase the pressure drop. If a separator and cyclone had similar dimensions and similar configuration, then they should have similar pressure drops. But the low cost, low tech reality of trash can separators is that we use 4" PVC pipe and 45 degree elbows.

My main belief is that plugged dust bags or canister filter are a major source of pressure drop and flow reduction. And that either a cheap cyclone or separator is an improvement over a plugged up canister. I have a Jet canister to sell if anyone is unconvinced.